As a conventional common mode choke coil, a common mode choke coil disclosed by Japanese Patent Laid-Open Publication No. 2006-261564 is known. In a common mode choke coil of this kind, while a signal is passing through the common mode choke coil, a differential-mode signal may be partly converted into a common-mode signal, and a differential-mode signal may be partly converted into a common-mode signal. (This is hereinafter referred to as mode conversion). In a differential transmission circuit for which such a common mode choke coil is used, the common-mode signal generated by the mode conversion becomes radiation noise, and the differential-mode signal generated by the mode conversion causes a malfunction of the circuit. Thus, such conventional common mode choke coils have a problem that radiation noise is caused or that the immunity of the circuit is lowered.